Social security administration states that if you file for your benefits late, they will pay you for the previous 6 months only. So, since you turned 66 in March and September is 6 months after March, if you file now, you will receive payments retroactively.

The information below was copied from the Social Security Administration website:

Caution: If you apply for benefits more than 6 months after the month you reach full retirement age, we can only pay benefits for the previous 6 months.

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